This application is related to U.S. patent application Ser. No. 10/990,004 filed: Nov. 16, 2004 and assigned to the same assignee as the present invention.
(1) Field of the Invention
This invention relates generally to the control of light emitting diodes (LED) currents, and more particularly to the control of the color and brightness of RGB LEDs.
(2) Description of the Prior Art
LED brightness control is typically achieved by controlling the current that passes through the LED. In order to dim LEDs with less power dissipation than current control, a method of power control is used known as Pulse Width Modulation (PWM). By varying the average current across the diode, the device can be made to appear dimmer or brighter or, in the case of RGB LEDs the color can be controlled.
The control of color and brightness of LEDs requires high PWM frequencies causing therefore high power dissipation compared to lower frequencies. A sole linear current digital-to-analog solution has the disadvantage of being perceived by human visual perception as a non-linear dimming.
There are various patents known dealing with the control of LEDs.
U.S. Pat. No. 6,586,890 to Min et al.) describes a driver circuit for light emitting diodes (LEDs) providing power to LEDs using pulse width modulation (PWM). The driver circuit uses current feedback to adjust power to LED arrays and provides a full light and a dim mode.
U.S. Pat. No. 6,596,977 to Muthu et al.) discloses an LED array being controlled by determining a constant relating the peak light output of an LED to the peak driving current of a PWM pulse driving the LED, and multiplying the average current of the PWM pulse by the constant to obtain a value of average light output for the LED. The constant may be determined by simultaneously measuring peak light output of the LED and peak current of a PWM pulse driving the LED. The constant is then calculated by dividing the peak light output by the peak current of the PWM pulse. By making the simultaneous measurements at a time during the duration of the PWM pulse where the pulse has reached its full magnitude, rise and fall times of the pulse do not affect the measurements. The average current of the PWM pulse may be determined by a variety of methods including integrating current in the PWM pulse over time, or passing the PWM current through a low pass filter configured for providing an average value of PWM current. Determining average current in this manner further reduces the effect of rise and fall time on determining the average light output of the LED.
U.S. Pat. No. 6,362,578 to Swanson et al.) teaches an LED driver circuit and method where an array of light emitting diodes has a transistor connected to each respective array of light emitting diodes. A PWM controller has an input for receiving a voltage reference and an output connected to selected transistors for driving selected transistors and setting a PWM duty cycle for the selected arrays of light emitting diodes to determine the brightness of selected light emitting diodes. An oscillator is connected to the PWM controller for driving the PWM controller.